In the related art, a technology of this field is disclosed in Japanese Published Unexamined Patent Application No. 2007-331601. In this disclosure, a mirror base including a base main body configured to support a mirror assembly (a door mirror main body) and a gasket installed at a back surface of the base main body is disclosed. The mirror base is installed between a door mirror attaching part formed at a vehicle body and a mirror assembly, and the base main body of the mirror base has a fixing part fixed to the door mirror attaching part. The fixing part protrudes toward the vehicle body in a horizontal direction, and butts against the door mirror attaching part via a gasket. Screws are threadedly inserted into a plurality of bosses formed at the fixing part of the base main body from the door mirror attaching part side, and thus the mirror base is fixed to the door mirror attaching part. In addition, a lower end of a door mirror opening and closing unit housed in the mirror assembly (the door mirror main body) is fixed to the base main body by a screw.